This invention relates to apparatus for and to a method of heat storage and, in particular to a method and apparatus in which bodies of heat storage medium at respective high and low temperatures are held in a tank with vertical separation due to their different densities. Such an apparatus and method may be used for example for bulk heat storage in a heating and cooling system for at least one building or a district.
JP-A-2-263026 describes apparatus, which employs a deep heat storage tank in an underground pit for storing high-temperature water at an upper section and low-temperature water at a lower section. A boundary float with apparent specific gravity set at a value half way between that of the hot and cold water is used to prevent heat transfer at the boundary between the hot and cold water.
JP-A-4-60325 describes heat storage using hot and cold water in a deep heat storage tank in an underground pit, storing in which high-temperature water (40.degree. C.) is stored at an upper section and low-temperature water (5.degree. C.) is stored at a lower section. To maintain the respective volumes of the hot and cold water, mixtures at 35.degree. C. and 10.degree. C., made by mixing hot water from the upper section of the tank and cold water from the lower section of the tank, are sent to a condenser and an evaporator respectively, of a heat pump. No float or other physical separator of the hot and cold water bodies is employed.
It is believed that systems as described in these prior art documents have not been built in practice. The operating costs of such systems increases when the volume of heat demand exceeds the amount of heat storage and creates a shortage of heat supply, for example, giving rise to a need to start a new heat output device to meet heat demand. This disadvantage particularly applies when the stored cold water, e.g. at 5.degree. C., is heated to provide the needed hot water. Conversely, excess heat storage is wasted when heat demand is smaller than the amount of heat stored. Also, in the case of the system disclosed in JP-A-4-60325, equal amounts of the system disclosed in hot and cold water are produced, by operation of the heat pump.
Furthermore, in the arrangement of JP-A-4-60325, hot water at the upper section of the tank and cold water at the lower section of the tank are stored in direct contact with each other. In this case, heat transfer is significant due to the large temperature difference, thus lowering the heat storage efficiency. In other words, heat loss occurs because the area of mixing of hot water and cold water cannot be used.
A float system, such as shown in JP-A-2-263026, will be likely to increase the construction and maintenance costs.